FIG. 1 illustrates an exemplary frame structure.
Referring to FIG. 1, a superframe (SF) includes a superframe header (SFH) and 4 frames F0, F1, F2 and F3. The frames in the superframe may have the same duration. While each superframe is 20 ms and each frame is 5 ms in FIG. 1, the sizes of the superframe and frame are not limited thereto. The duration of a superframe, the number of frames included in a superframe, and the number of subframes included in a frame may vary. The number of subframes included in a frame may depend on channel bandwidth, the duration of a cyclic prefix (CP), etc.
A frame includes a plurality of subframes SF0, SF1, SF2, SF3, Sf4, SF5, SF6 and SF7. Each subframe may be used for uplink or downlink transmission. A subframe includes a plurality of orthogonal frequency division multiplexing (OFDM) symbols or orthogonal frequency division multiple access (OFDMA) symbols in the time domain and includes a plurality of subcarriers in the frequency domain.
An OFDM symbol represents a symbol period and may be referred to as an OFDMA symbol, SC-FDMA symbol, etc. according to multiple access scheme.
While a subframe is composed of 5, 6, 7 or 9 OFDMA symbols, the number of OFDMA symbols included in a subframe is not limited. The number of OFDMA symbols included in a subframe may depend on channel bandwidth, the duration of a CP, etc.
The type of a subframe may be defined according to the number of OFDMA symbols included in the subframe. For example, a type-1 subframe includes 6 OFDMA symbols, a type-2 subframe includes 7 OFDMA symbols, a type-3 subframe includes OFDMA symbols, and a type-4 subframe includes 9 OFDMA symbols. One frame may include subframes of the same type. Otherwise, one frame may include subframes of different types. That is, subframes included in a frame may have the same number of OFDMA symbols or different numbers of OFDMA symbols. Otherwise, the number of OFDMA symbols included in at least one subframe in a frame may differ from the number of OFDMA symbols of other subframes in the frame.
Time division duplexing (TDD) or frequency division duplexing (FDD) may be applied to frames. Subframes are used for uplink transmission or downlink transmission at the same frequency in different time periods in the TDD scheme.
That is, subframes in a TDD frame are divided into uplink subframes and downlink subframes in the time domain. In the FDD scheme, subframes are used for uplink transmission or downlink transmission at different frequencies in the same time period.
That is, subframes included in an FDD frame are divided into uplink subframes and downlink subframes in the frequency domain. Uplink transmission and downlink transmission may be simultaneously performed while respectively occupying different frequency bands.
An SFH may carry an essential system parameter and system configuration information. The SFH may be located in the first subframe of a superframe. The SFH may occupy the last 5 OFDMA symbols in the first subframe.
The SFH may be classified into a primary SFH (P-SFH) and a secondary SFH (S-SFH). The P-SFH and S-SFH may be transmitted in each superframe. The S-SFH may be transmitted in two contiguous superframes. Information transmitted through the S-SFH may be divided into 3 sub-packets S-SFH SP1, S-SFH SP2 and S-SFH SP3. The sub-packets may be periodically transmitted at different intervals. Information transmitted through the sub-packets S-SFH SP1, S-SFH SP2 and S-SFH SP3 may have different degrees of importance. S-SFH SP1 may be transmitted at the shortest interval and S-SFH SP3 may be transmitted at the longest interval.
S-SFH SP1 includes information about network re-entry. S-SFH SP2 includes information about initial network entry and network discovery. S-SFH SP3 includes other important system information.
An OFDMA symbol includes a plurality of subcarriers, and the number of subcarriers is determined according to FFT size. Subcarriers may be classified into a data subcarrier for data transmission, a pilot subcarrier for various estimations, and a null carrier for a guard band and DC carrier.
Machine to Machine (M2M) Communication
Machine to machine (M2M) communication will now be described.
M2M communication refers to communication between electronic devices. In a broad sense, M2M communication means wired or wireless communication between electronic devices or communication between devices controlled by people. Recently, M2M communication generally refers to wireless communication between electronic devices, performed without human intervention. Performance or capability of M2M terminals used in a cellular network is poorer than that of general terminals.
An M2M environment has the following characteristics.
1. A large number of terminals per cell
2. A small quantity of data
3. Low frequency of transmission
4. A limited number of data characteristics
5. Insensitiveness to time delay.
Many terminals are present in a cell and they may be discriminated by type, class, service type, etc.
Particularly, the number of terminals may abruptly increase when M2M communication (or machine type communication (MTC)) is considered. M2M terminals may have the following characteristics according to services supported thereby.
1. M2M terminals intermittently transmit data. The M2M terminals may have periodicity.
2. The M2M terminals have low mobility or are fixed.
3. The M2M terminals are insensitive to latency in signal transmission.
M2M terminals having the above characteristics in a cell can transmit or receive signals to/from a base station or other terminals using a multi-hop configuration or a hierarchical structure.
That is, an M2M terminal may receive a signal from the base station and transmit the received signal to an M2M terminal located at a different layer or a lower layer, or receive a signal from other M2M terminals and transmit the signal to other M2M terminals or the base station. Otherwise, direct communication between M2M terminals may be performed without using a relay.
For signal transmission between M2M terminals, the M2M terminals may be connected in an upper/lower structure to transmit signals (although the upper/lower structure may not be employed in the case of direct communication between terminals, signal transmission may be described by applying the upper/lower structure to the direct communication between terminals).
In downlink transmission, for example, mobile station (MS) 1 receives a signal from the base station and transmits the received signal to MS 2. Here, MS 1 may transmit the signal to a lower MS as well as MS 2. MS 2 is a lower terminal of MS 1.
Upon reception of the signal from MS 1, MS 2 transmits the received signal to a lower MS. In this manner, the signal is transmitted to MS N. In this case, many terminals may be connected in a multi-hop or hierarchical structure between MS 2 and MS N.
Alternatively, in uplink transmission, signal transmission between M2M terminals may be performed as follows. A lower M2M terminal may transmit a signal to another M2M terminal or the base station using a higher M2M terminal.